Death penalty opponents hold a vigil outside St. Francis Xavier College Church in St. Louis Tuesday. / Jeff Roberson, AP

by Richard Wolf, USA TODAY

by Richard Wolf, USA TODAY

WASHINGTON -- The Supreme Court blocked the execution of a convicted Missouri murderer Wednesday evening rather than risk another in what has become a series of botched lethal injections.

The ruling marks a stark reversal from the court's practice of denying last-minute appeals by death row inmates. That could mean a majority of justices are concerned about a shortage of drugs that has forced states to rely on unregulated compounding pharmacies. Some states have refused to say where they get their drugs or specify which drugs are used.

Lawyers and a doctor for Russell Bucklew, 46, convicted in 1996 for killing a man who lived with his ex-girlfriend, then kidnapping and raping her, argued that a birth defect affecting his veins could turn the lethal injection into a fiasco.

Supreme Court Justice Samuel Alito halted the process late Tuesday night, giving both sides the opportunity to file arguments on the day of Bucklew's scheduled execution. Alito's action followed a decision by the full U.S. Court of Appeals for the 8th Circuit to let the execution go forward. That same court now will reconsider his case.

Bucklew's impending execution heightened the nationwide debate over how condemned prisoners are put to death. The last one to die, Oklahoma inmate Clayton Lockett, writhed in pain before dying of a heart attack 43 minutes later on April 29.

Lethal injection, the method favored by states using capital punishment, has become more controversial as pharmaceutical companies increasingly refuse to supply the necessary drugs. A January execution in Ohio caused the prisoner to snort and gasp for breath.

After the botched Oklahoma execution, President Obama said his administration would look at the steps taken to address what he called "some of the significant problems" in how the death penalty is applied. "I think we do have to, as a society, ask ourselves some difficult and profound questions around these issues," he said.

Missouri corrections officials had said their chemical cocktail would be more effective than the one used in Oklahoma. But Bucklew's lawyers argued that his massive vascular tumors could cause "hemorrhaging in face, head or throat, coughing or choking on his own blood, suffering a total airway obstruction and suffocating to death, and/or suffering an excruciating and prolonged execution because the lethal drug does not properly circulate in his body."

Bucklew had told The Guardian newspaper this month that he was fearful about what he would experience. "I'm sick about it not working on me," he said. "I'm afraid that it's going to turn me into a vegetable."

The state countered that since Bucklew has known about his medical condition for decades and has not suggested a feasible alternative that would be more humane, the court should deny his request.

Twenty prisoners have been executed in the U.S. this year, including four in Missouri. Texas leads the nation with seven executions, followed by Florida with five. The others were in Oklahoma and Ohio.

The Supreme Court was asked to intervene in Missouri's three other executions in January, February and March. The court eventually let each one go forward, but growing numbers of justices dissented. In the most recent case and another in Texas, all four liberal justices said they would have granted a stay of execution.

The court earlier this year refused to consider a broad challenge to the constitutionality of states' lethal injection methods following complaints about a lack of transparency.

Thirty-two states still have capital punishment on the books, but in 2013 only nine states carried out 39 executions. Six states -- New York, New Jersey, Illinois, Maryland, Connecticut and New Mexico -- have abolished the procedure in recent years: Governors in Colorado, Oregon and Washington have announced a halt to executions.